Volume control system



Oct. 31, 1933. w. H. T. HOLDEN VOLUME CONTROL SYSTEM Filed Jan. 6, 1931INVENTOR Wfilfi oldel a ATTORNEY Patented Got. 31, 1933 OFFICE I *1,VOLUME ooN'rnoL SYSTEM William H. T. Holden, Brooklyn, N. Y., assignorto American Telephone and Telegraph Company, a corporation of New YorkApplication January 6,

3 Claims.

This invention relates to signaling systems. More particularly, thisinvention relates to apparatus for the control of the gain of the energytransmitted through signaling systems. Still more particularly, thisinvention relates to gain control apparatus embodying screen grid tubes.

This invention includes a form of automatic volume control apparatusadaptable to arrangements employing screen grid vacuum tube ampliiiers.Such apparatus maybe so employed, for it has been known for some timethat any variation of the screen voltage furnished an effective gaincontrolling means for the amplifiers employing the screen grid tubes.Ihis invention is adaptable to single or multi-channel systems, and ishere shown in one form as a single channel radio receivezv I Thisinvention will be better understood from the detailed descriptionhereinafter following when read in connection with the accompanyingdrawing in which Figure 1 represents one embodiment of the inventionshowing how the gain of an amplifier having a screen grid tube may beautomatically controlled, and Fig. 2 shows a radio receiver embodyingmany of the features shown in Fig. l and still further features.

Referring'to Fig. 1 of the drawing, the reference characters A1 and A2will designate two tubes forming an amplifying system interconnecting aninput circuit K1 and an output circuit K2 through transformers T1 andT2, respectively. These tubes are made to act as amplifiers, tube A2being of the simplethree-electrode type and tube A1.of the screen gridtype. In this amplifying system a condenser C1 :connects the plateelectrode of the tube A1 to the grid electrode of the tube A2. Moreover,the plate electrode of the tube A1 is supplied with positive operatingpotential by a battery B1 through a winding L1. Furthermore, the gridelectrode of the tube A2 is connected to its'filament electrode througha resistance R1 and 'a battery If additional stages of amplification arerequired, tubes similar to A1 may be connected in tandem in the circuitin any well known manner, all of the screens, however, being connectedin parallel relationship.

A portion of the energy in the outputcircuit K2 is transmitted to arectifier or detector D through a transformer T3 and an amplifier As, ifone is found necessary or desirable. The output circuit or" the detectorD may be considered to be composed of a battery B3, a resistance R2, awinding L2, the plate and filamentgelectrodes of the tube D and ground.The battery B3 supplies normal positive operating potential for theplate 1931. Serial No. 506,959

electrode of the tube D. This plate electrode is grounded thru acondenser C2.

It is to be noted that the current in the output circuit of the detectorD is normal when the level of the energy flowing through the outputcircuit K2 is at a predetermined value or, finctuates between certainclosely spaced predetermined levels. Any rise in the level of the energyflowing through the output circuit K2 will cause a correspondingincrease in the current of the output circuit of the detector D aboveits normal value. Similarly, any decrease in thelevel of the energy ofthe output circuit K2 will cause a corresponding decrease in the currentof the output circuit of the detector D below its normal value. It is tobe here noted that the winding L2 and the condenser C2 form a section ofa low pass wave filter which may have many more sections having similarseries windings and shunt condensers, and this filter will suppress suchalternating. currents as may tend to flow through the output circuit ofthe tube D.

The screen electrode of the tube A1 is connected to ground through acircuit which includes a winding La,v resistance R2 and battery B3. Thisscreen'electrode is also grounded through a condenser C3. By virtue ofthe connection of the screen electrode inseries with resistance R2 andbattery B3, the voltage applied to the screen electrode will bedetermined by the normal voltage of the battery B3 and by the drop involtage across the resistance R2. It is to be noted that the screenelectrode is normally positive with respect to the filament electrode ofthe tube A1 and yet it is negative with respect to the plate elec- 90trode of this tube and remains negative with re spect to this plateelectrode at all times. The condenser C3 is one of large capacity,thereby providing a path of low impedance for all the operatingfrequencies.

The drop in voltage across the resistance R2 corresponds to the flow ofcurrent through the output circuit of the tube D. The voltage 'dropacross the resistance R2 will increase when the current of the outputcircuit of the tube D increases, and conversely, this voltage willdecrease when the current through the output circuit of the tube Ddecreases; As the drop in voltage across the resistance R2 increasesabove its normal value, the voltage of the screen electrode of the tubeA1 will be correspondingly decreased, thereby reducing the amplificationof the tube A1. Moreover, as the drop in voltage across the resistanceR2 decreases below its normal value, the voltage of the screen electrodeof the tube A1 will 110 be correspondingly increased, thereby increasingthe amplication of the tube A1.

The winding L2 and the condenser 02 not only act as a section of a wavefilter, but their re" spective magnitudes are such that they provide atime constant which will prevent the too rapid readjustment of the gainof the system. Any too rapid readjustment of the gain is undesirablebecause it will affect the quality of the signals upon theirdemodulation.

A radio receiver completely operated by alternating current is shown inFig. 2 of the drawing. Here the reference characters V1,'V2, V3, V4 andV designate a plurality of tubes employed to translate radio frequencysignals which may become impressed upon an antenna N into voicefrequency fluctuations which may be rendered audible by a loud speakerLS. The tube V1 is coupled to the antenna circuit through a transformerT1. The tube V1 is also coupled to a tube V2 through a transformer T2.The tube V2 is coupled to a tube V3 through a transformer T3. The tubesV1 and V2 are screen grid tubes of the heater type and act as amplifiersof radio frequency currents. The secondary windings of the transformersT1, T2 and T3 are bridged by variable condensers C1, C2, and C3,respectively, and these condensers may be adjusted so as to selectivelytransmit such radio frequencies as are impressed upon the antenna N andare desired. Obviously, the variable elements of these condensers may bemounted on a common shaft so that their reactive values may be changedby the same amount simultaneously. Condensers C4 and C5 are shown inshunt with the condensers C1 ad 02 and are provided for the purpose ofcompensating for the additional capacity across the tuned circuitworking into the tube V3 in order that these circuits may besimultaneously tuned.

The tube V3 is employed to act as a detector in the arrangement shown inFig. 2. This tube is coupled to a tube V4 through a transformer T4. TubeV4 is similarly coupled to a tube V5 through a transformer T5. Atransformer Ts couples the output circuit of the tube V5 with the loudspeaker LS. The tubes V4 and V5 are employed to act as audio frequencyamplifiers, and the transformers T4, T5 and T6 may be of the iron coretype, as shown. The tube V5 may be any form of power amplifier.

Tubes V6 and V7 form the essential devices of the automatic vacuumcontrol apparatus of this invention. Tube V6 is an auxiliary amplifierof the screen grid and heater type, the grid electrode of which isconnected in parallel relationship With the grid electrode or the tubeV3. Tube 'V1 is of the ordinary three-electrode type and is detailhereinafter.

The potentials required to energize the electrodes of the various vacuumtubes are derived from a source of alternating current designated S.This source is connected to the primary winding of a transformer T1, thesecondary of which includes windings W1, W2,v W3, W4 and W5. A tubedesignated Va is used for the purpose of providing rectification, andthis tube includes a filamerit and two plate electrodes. The plateelectrodes of this tube are connected to the terminals of the secondarywinding W1, the midpoint of which is grounded. The secondary winding W2is connected to the filament of the tube Va, this winding supplying thecurrent required to heat the filament of this tube to incandescence. Thecircuit receiving rectified current derived from the tube V8 includesresistances R1 and R2, windings L1 and L2, the filament of the tube Vs,both of the plate electrodes of the tube Va, both halves of thesecondary winding W1 and ground. The terminal common to the Windin L1and the resistance R2 is grounded through a condenser Cs; the terminalcommon to the windings L1 and L2 is grounded through a condenser C1; theright hand terminal of the winding L2 is grounded through a condenserC8; and the terminal common to the resistances R1 and R2 is groundedthrough a condenser C9. It is to be noted that the windings L1 and L2and the condensers C6, C7 and Ca form a pair of sections of a low passWave filter which is designed to suppress such alternating currents asmay tend to pass through the circuit supplied with rectified current bythe tube V2.

The filaments of the tubes V1, V2, V3, V4 and V6 are connected inparallel relationship with the secondary winding W4. The filament of thetube V5 is connected to the winding W5, the midpoint of which isconnected to ground through aresistance R3 and a condenser C10 which arearranged in parallel relationship. The filament of the tube V7 isconnected to the secondary winding W3 and it is also shunted by aresistance R4,

and yet it will be understood that the filament of this tube need not beof a type which is indirectly heated.

The heaters of the tubes V1, V2, V3, V4 and V6 are respectivelyconnected to-ground through resistances R5, R6, R7, R3 and R9. Theseresistances are shunted by condensers C11, C12, C13, C1 1 and C15. Theseresistances are common to the output circuits of the respective tubes,and moreover, they are in effect connected between the grid electrodesof these tubes and their heaters, and'provide the necessary biastherefor. The shunting condensers by-pass alternating currents aroundthese resistances.

The necessary positive operating potentials for the plate electrodes ofthe tubes V1 to V1, inl elusive, are provided by the drops in voltageacross the resistances R1 and R2. The right-hand ter-- minal of theresistance B1 is connected to the plate electrode of the tube V1throughthe winding L3 and the primary winding of the transformer T2.This same terminal of the resistance R1 is also connected to the plateelectrode of the tube V2 through a winding L4and the primary winding ofthe transformer T3. This same terminal is also connected to the plateelectrodes of the tubes V3 and V4 through the primary windings of thetransformers T and T5," respectively,'and moreover, it isdirectlyconnected to the plate electrode of the tube V7. It is also connected tothe plate electrode of thetube Vathrough a winding L5, The right hand.terminal of the resistance R2 is connected to the plate electrode of thepower tube V5 through the primary winding of the tral sformer T6.

voltage drop across the resistance R1 may, for

example, be about 180 volts, and that across both resistances R1 and R2may, for example, be about 400' volts. Both of these resi tances may, ifdesired, be variable in their respective magnitudes in order to providethe desiredpositive operating potentials for the plate electrodes of thevarious tubes. It is to be noted that the winding L5 is a choke coil andcouples the tubes V6 and V1, thereby avoiding the necessity for tuningthe system at this point.

The tube V7 is arranged to operate as a grid leak detector and as suchits plate current is reduced and the resistance between its plate andfilament electrodes increased when alternating current is applied to itsgrid electrode. Here a relatively large condenser C16 connects the plateelectrode of the tube V6 with the grid electrode of the tube V7, and ahigh grid leak resistance R10 is interposed between the grid electrodeof the tube V7 and the midpoint of the resistance R4. It is to be notedthat the lower terminal of the resistance R10 is connected to groundthrough a winding L6 and a resistance R11. The opposite terminals of thewinding Ls are grounded through condensers C17 and C18. Winding LG andcondensers C17 and C18 form one of the sections of a low pass wavefilter which may have a cut-off frequency of about ten cycles. Thecapacity of the condenser C16 is large and the resistance of the leakR10 is high so as to eliminate from the detector V7 frequencies abovethe cut-off frequency of the filter having the elements composed ofwinding L6 and condensers C17 and C111, all of which tend to reduce theattenuation .required in the detector.

The midpoint of the resistance R4 is connected to the screens of thetubes V1 and V2 through winding L6 and individual windings L7 and L3,respectively. These screens are also grounded through condensers C19 andC20, respectively. The right-hand terminal of the resistance R1 isconnected to ground through resistances R12 and R13. The terminal commonto these'resistances is connected to the screen of the tube V6. Thisscreen is also grounded through a condenser C21.

The windings L7 and L8 and the condensers C19 and C20 prevent anyintercoupling of the output circuits of the tubes V1 and V2. It will beunderstood that the condensers C22 and C23 are connected, respectively,in the output circuits of the tubes V1 and V2 in series with the primarywindings of the transformers T2 and T3, and that these condensers freelytransmit all radio frequencies amplified by the tubes V1 and V2.

A circuit deserving special study is the one including the resistanceR1, the plate and filaments electrodes ofthe tube V7, the resistance R4,the winding Ls, the resistance R11 and ground. In shunt with theresistance R11 are two parallel paths including windings L7 and L8 andthe screens of the tubes V1 and V2, respectively. The

resistance between the plate and filament electrodes of the tube V7 andthe resistance R11 are the largest impedances in the series circuit justdescribed, and it may be assumed for this illustration that the totalvoltage across the resistance R1 is divided therebetween in accordancewith their relative magnitudes. This is especially true when it isappreciated that the screens of the tubes V1 and V2 receivesubstantially no currents and are affected merely by voltage changes. Ofcourse, any change in the magnitude of the resistance R11, which may beaccom- 1 plished manually in any manner well known in the art, willpermit the screens of the tubes. V1 and V2 to be normally supplied withany predetermined positive potentials. The normal positive potentialsfor these screens may be considered as those potentials impressedthereon when no alternating current reaches the input circuit of thetube V7.

When the various radio frequency stages of the receiving system shown inFig. 2 are tuned to the frequency of some desired carrier current, theimpression of this current on the input circuit of the tube V7 will, ifof sufficient magnitude, cause an increase in the resistance between theplate and filament electrodes of the tube V7. The greater the currentimpressed upon the input circuit of the tube V7 the greater will be theresistance between the plate and filament electrodes of the tube V7.Each increase in resistance between these electrodes will reduce thegain of the tubes V1 and V2. Generally, the greater the resistancebetween the plate and filament electrodes of the tube V7 the lower willbe the gain of the tubes V1 and V2.

The screen of the tube V6 is supplied with a positive potentialdetermined by the drop in voltage across the resistance R13. The voltageapplied to this screen is constant and, therefore, the gain of the tubeV6 remains unchanged.

While this invention has been shown and described in certain particulararrangements merely for the purpose of illustration, it will beunderstood that the general principles of this invention may be appliedto other and widely varied organizations without departing from thespirit of the invention and the scope of the appended claims.

What is claimed is:

l. The combination of a vacuum tube grid leak detector, a resistance, asource of direct current potential connected in series with saidresistance and the plate and filament electrodes of said detector, and aplurality of tubes all having screen electrodes which are connected inparallel relationship, said parallel connected screen electrodes beingin parallel with said resistance and in series with said source ofdirect current potential and the plate and filament electrodes of saiddetector.

2. The combination of a vacuum tube grid leak detector, a resistance, asource of direct current potential connected in series with saidresistance and'the output circuit of said detector, a source ofalternating current coupled to the input circuit of said detector, and aplurality of amplifiers all having screen electrodes which are connectedin parallel relationship, the parallel arrangement of said screenelectrodes being con-' nected in parallel with said resistance and inseries with said source of direct current potential and the outputcircuit of said detector.

3. The combination of a source of rectified direct current potential, avacuum tube grid leak detector having an input circuit and an outputcircuit, the output circuit of said detector having a space dischargepath, a resistance connected in series with said source of directcurrent potential and the output circuit of said detector, a pluralityof amplifiers each having a screen electrode, the screen electrodes ofall of said amplifiers being connected in parallel relationship, theparallel screen electrodes being connected in parallel with respect tosaid resistance, a source of signals coupled to the input circuit ofsaid detector, and means for preventing the source of signals fromimpressing alternating voltages on the scree electrodes of saidamplifiers.

WILLIAM H. T. HOLDEN.

